Machines for tunnelling through the earth on a continuous basis are known, and examples of these include continuous mining machines which are used to mine coal. Typically, these machines employ a rotating cylindrical cutting head which cuts into the coal so that this falls away in chunks or blocks, and these are then transported out of the tunnel by a conveyor belt or other means.
Machines of this type have proven highly successful for mining coal, which is a relatively soft and non-abrasive material, but they are generally incapable of tunnelling through rock or most types of soil. In addition to rock, examples of such soils include ordinary earth, as well as relatively more solid material such as cemented conglomerate and permafrost. For purposes of illustration, the following description will discuss application of the mining machine of the present invention to excavation of permafrost; however, it will be understood that the present invention is not limited to this particular application, and that the structure discussed below and its operation and advantages apply with equal force to the excavation of other earth and materials, or even the demolition of buildings or other structures.
Permafrost soils occur widely in the polar regions of the world. The composition of the permafrost soil itself varies somewhat, but commonly this is a frozen alluvial deposit consisting of gravel or cobbles suspended in a matrix of clay and ice, with occasional large boulders being interspersed in this. Various operations undertaken in arctic regions require tunnelling through this material, including both mineral mining and petroleum production, and conventional continuous mining machines have proven wholly unsuitable for this. In large part, this is due to the compressive strength and extreme abrasiveness of the materials (e.g., the quartz gravel and frozen clay) which make up the permafrost soil, and furthermore the large boulders which are frequently encountered in the permafrost are utterly beyond the capabilities of such machines.
In addition to the continuous mining machines described above, machines referred to as "roadheaders" are also known to those skilled in the tunnelling art, these typically being used to remove rock faults and other areas of hard material which are encountered in coal mining. Typically, these machines include a chassis on which a turntable is mounted for rotation in the horizontal plane, this being achieved by means of hydraulic slew rams which are connected between the turntable and the chassis. The turntable assembly supports a boom which is mounted for pivotal movement in a vertical plane, and this carries a rotating conical head having a scroll formation of "picks" arranged on it. An example of a roadheader of this type is the Mk2B series machine, available from the Dosco Corporation, Route 10, Box 324, Abingdon, Va., U.S.A.
While machines of this type are quite capable of removing many obstacles formed of rock or other very hard material, they are not suited to continuous excavation of a tunnel at a relatively high rate of speed. Furthermore, they are not suitable for excavation of soil materials such as the permafrost material described above. For example, the rotating head assembly is susceptible to excessive wear in the highly abrasive environment which is posed by permafrost and similar soils, and the head also tends to reduce or "chew" such material into unnecessarily small particles, which represents excessive energy consumption. Perhaps more fundamentally, the high-speed rotating cutter head of these machines is only effective against a competent face, and will tend to bounce off of boulders when these become dislodged from the soil, rather than breaking them up; furthermore, should the rotating cutter head encounter a firmly lodged boulder while working through the surrounding matrix, the impact may actually tear the picks off of the head and destroy it.
Accordingly, a need exists for a machine to excavate tunnels through rock and soils at a relatively high rate of speed, which machine will obviate the numerous disadvantages which are exhibited by known types of continuous mining machines with respect to such applications.